In the production of blown film, it is desirable to provide a uniform flow of cooling air to the film bubble as it emerges from the extruder die. Typically the film bubble travels vertically upward from the die and the ring is axially aligned with the bubble. In the case of external air rings, the air ring is horizontal, of annular form and surrounds the bubble.
Nonuniformity in the flow from air rings tends to cause undesirable nonuniformities in the film. Cooling air is generally supplied through discrete conduits to the air ring and the degree of uniformity of the film that can be obtained depends upon the degree to which the nonuniform pattern of the cooling air flow produced by the discrete conduits has been eliminated by the air ring.
For external air rings it is common for the conduits to introduce air in a tangential flow direction into an annular plenum of the air ring to produce a swirling flow, and then this air is directed to proceed inwardly toward the bubble through a flow-evening path.
It is also known to guide the major stream of cooling air in a generally axial direction along the surface of the film bubble and to direct a minor air stream against the film, at a point adjacent to the extruder die, prior to the major stream, with the minor stream flowing between the film and the major stream guide lip.
Some of the known apparatus have required wasteful amounts of energy to operate because of their high flow resistance. Further, their characteristic high flow resistance and high air velocities make the designs sensitive to small circumferential differences in flow path dimensions, as such differences produce circumferential nonuniformities of flow which become accentuated at high velocities. In one distributing ring, for example, a flow-evening path in the cooling stream includes narrow slots which are spaced circumferentially about the ring. The narrow slots offer relatively high flow resistance.
In todays's industry there is a desire to operate the film extrusion line at greater speeds than heretofore. Such speeds place increased demands upon the flow capacity of the air cooling equipment. As flow velocities and flow rates are increased, previously acceptable air-distributing ring designs, in scale-up of size, increasingly show their deficiencies. They tend to produce nonuniform air distribution patterns, they require larger and noisy blowers, and they tend to be cumbersome in size and expensive to manufacture.
To overcome these problems, thin, radial fin elements have been employed, located with their leading edges in a concentric circle to receive air from the plenum. These fin elements serve to direct the inner portion of the swirling air mass in the plenum radially toward the outlet at the film bubble. Such fin arrangements, however, are expensive and may not fully alleviate turbulence.
It is an object of the present invention to provide a relatively inexpensive air ring construction which overcomes these problems to deliver the volume of cooling air uniformly to the film bubble.